In an optical disk reproduction system, a photodiode senses light transmitted to a disk by an optical pickup and converts the sensed, reflected optical signal into an analog RF signal. The analog RF signal output of the photodiode is provided to an asymmetry correcting apparatus, which corrects the asymmetry of the RF signal and converts the RF signal into a digital eight-to-fourteen modulation (EFM) signal. Namely, the asymmetry correcting apparatus slices the analog RF signal on the basis of an asymmetry-corrected slice reference level, and obtains the digital EFM signal. An example of such an asymmetry correcting apparatus is provided in `EFM comparator (CXA12710)` (hereinafter, referred to as a conventional asymmetry correcting apparatus) described at page 89 of the "SONY SEMICONDUCTOR IC DATABOOK" published in 1990 by SONY.RTM..
Hereinafter, the structure and operation of the conventional asymmetry correcting apparatus will be described with reference to the attached drawings.
FIG. 1 is a circuit diagram of a conventional asymmetry correcting apparatus. The apparatus includes capacitors C1, C2, and C3, a comparator 10, operational amplifiers 12 and 14, resistors R1, R2, R3, R4, R5, and R6, and inverters 11 and 12, with Vcc signifying a power supply.
The operational amplifier 14 shown in FIG. 1 operates as an automatic asymmetry buffer. Resistors R1, R2, R3, and R4 and the operational amplifier 12 operates as an automatic asymmetry control amplifier. A low pass filter (LPF) 16, which can be connected externally, for detecting a direct current (DC) offset, is comprised of resistors R5 and R6 and capacitors C2 and C3. A CMOS buffer 18 is comprised of inverters I1 and 12.
Capacitor C1 shown in FIG. 1 eliminates the direct current component of the RF signal input via input terminal IN1, from an optical diode (not shown). The comparator 10 compares the alternating current (AC) component of the RF signal input at the positive input terminal thereof with a reference signal output from the operational amplifier 12, and outputs the comparison result through an output terminal OUT, as a digital EFM signal. The digital EFM signal is buffered by the CMOS buffer 18, and passes through the low pass filter 16, the asymmetry buffer 14, and the automatic asymmetry control amplifier 12, and is input to the negative input terminal of the comparator 10 as a reference signal.
A disk defect such as a scratch or hole is caused where the material of the disk is so poor, or the disk is so deeply gouged, that light passes through the disk without being reflected. Accordingly, the data is not detected. In the above-mentioned conventional asymmetry correcting apparatus, when the optical disk has such a defect, it can take considerable time, for example the time constant of the slice reference level, which is the level of the reference signal output from the operational amplifier 12, to determine the middle level of the RF signal. For example, since the time constant of the low pass filter 16, including resistors R5 and R6 and capacitors C2 and C3 is, for example, 5 ms, it is difficult for the slice reference level to track the middle level of the RF signal in the portion of the signal having a scratch or hole defect, which is much shorter than 5 ms in duration. Therefore, since the asymmetry of the RF signal is not properly corrected, systems which depend on an asymmetry-corrected signal, for example an error correction circuit are likely to malfunction.